Carnivorous Plants

Above: - illustration of a Nepenthese pitcher. The pitchers are modified leaves and are beautiful structures and sometimes quite large!

Pitcher Plants

Pitcher plants are extraordinary carnivorous plants. They have modified leaves that form special receptacles that contain fluid. Insects and other small animals (and also plant debris) that fall into the pitcher are broken down and absorbed. In particular, the nitrogen obtained in this way is a valuable nutrient as nitrogen often limits plant growth and carnivorous plants often occur where nitrogen is in short supply. The pitchers can be quite large and dead rats are occasionally found inside the very largest. Some pitchers secrete enzymes and acid to digest their prey, whilst others rely on bacterial decomposition or both. The enzymes that may be secreted include proteases (digest proteins) and carbohydrases (digest carbohydrates like starch). Digestion is typically slow and to stop the animals escaping various tactics are employed. Downward pointing hairs or spines inside the pitcher, combined with a slippery, waxy surface make it hard for insects to crawl back out!

Darlingtonia, the California pitcher or cobra plant has particularly fascinating pitchers that look like snakes, with their forked protruding appendage and curved over hoods. A spiral passage connects the digestion chamber to the outside world and even if trapped insects can navigate this, the hood has a special trick to deceive them - it contains fenestrae or translucent windows which allow light through. The insect makes a line for these windows, believing them to be exits, flies up, bumps into the hood and falls back down the spiral shoot into the trap! This compensates for the fact that Darlingtonia does not secrete digestive enzymes, but relies on the slower process of bacterial decay.

Chance encounter might not be sufficient and pitchers have ways of luring their prey!
Sarracenia purpurea, a pitcher from NE America has a green pitcher which is red at the top and has nectaries around the aperture and releases a violet odour. The colour, promise of food and odour all serve to attract insects. Most of these insects are actually not flies but largely crawling insects and Sarracenia has a single ventral ridge the ala ventralis (ventral wing) which is a causeway along which insects can crawl up the pitcher to the slick, waxy surface of the rim (peristome) - and down they fall! Rows of downward pointing hairs, the slippery inside surface and the viscosity of the fluid in the trap all make it hard for the insects to get back out. The nectaries secrete an alkaloid narcotic, in addition to sugar, to anaesthetise the insect, again making it hard for the animal to avoid slipping in! Nepenthes has nectaries around the waxy rim and large downward-pointing teeth inside the tube and secretes digestive enzymes. Nepenthes is a tree-climbing vine.

Nepenthes

Above: Nepenthes (diagram based upon the type description and figures of Nepenthes pantaronensis in Gronemeyer et al. 2014. Plants 3: 284-303). Species of Nepenthes are extremely variable in pitcher form. (External link: http://specialflowersintheworld.blogspot.com/2015/04/nepenthes-flower.html).

Nepenthes species use a variety of trapping mechanisms to catch prey. In some the peristome (the coloured rim of the pitcher) is a wettable surface which easily becomes coated with a film of rain water or nectar, forming a slippery surface. Insects alighting on the rim easily slip into the pitcher (wet capture). In others, the main mechanism is the secretion of slippery wax crystals on the inside of the pitcher tube. These plate-like waxy crystals easily detach, sticking to the adhesive pads of insect feet, contaminating them and making them more slippy dry capture). Additionally, the wax seems to neutralise the sticky secretion of the insect's feet. Insect feet (tarsi) have two primary adhesive mechanisms: tarsal claws and adhesive pads (see insect locomotion). Both mechanisms must be overcome for an insect to slip. Downward-pointing hairs (and the downward-pointing epidermal cells) on the inside pitcher wall make it harder for the insect/arthropod to crawl back up and escape the trap. Nepenthes rafflesiana has different varieties, with different pitcher forms, one relying on peristome capture, the other on a slippery inner pitcher wall (10).

Nepenthes gracilis relies primarily on the lid, the undersurface of which secretes most of the nectar and is coated in semi-slippery crystals of wax. An arthropod, such as an ant, stretching across the opening to reach the nectar may be flicked into the trap by falling rain! Further, the digestive fluid of the trap may be highly viscous and sticky and may contain alkaloids which anaesthetise the prey. (8)

Nepenthes aristolochioides has a pitcher with a translucent dome which acts as a light trap. Experiments have shown that light transmitted through the pitcher dome is important in attracting and trapping small flies, such as fruit flies (Drosophila). (9)

Finally, many pitchers are detritivores rather than insectivorous, collecting and digesting falling plant debris. 


Utricularia (bladderwort)

Not all carnivorous plants use mechanisms as passive as those of pitcher plants. The next example we look at are the aquatic bladderworts, or utricularias. These have extremely elaborate trap mechanisms!

Left a bladder of Utricularia. Utricularia is a freshwater aquatic plant which lacks roots and floats in the water. It gives off stalk-like stolons which bear small bladders, which contain fluid under reduced pressure, each no more than 5 mm long.

These bladders are sophisticated traps, primed and ready to catch prey, which is typically small crustaceans, like water fleas and copepods, and insect larvae. These creatures try to hide amongst the hairs (trichomes) that project from the bladder (borne on wings (W) in the example on the left) but these hairs are primed triggers and when the prey brushes against them the trap springs open! Water rushes into the bladder, sweeping along the prey with it. The trap then seals shut and enzymes digest the unfortunate victim! R, rostrum; S, stalk; W, wing.

Utricularia bladder

Below: a longitudinal section through one of the bladders. The trap is primed and a flap-like valve (V) closes the entrance. This valve is wedged shut. Pulling on the trichomes (T) springs the trap, the valve opens and water rushes in. Inside the bladder lining possesses small glandular hairs or internal glands (IG), some of these have four cells at their tips (4 end-branches) and are called quadrifid glands, whilst some have two terminal cells and are called bifid glands. The outside of the bladder contains equidistantly spaced small spherical or nipple-like glands (external glands). The functioning of these glands are described below.

Utricularia bladder TS

Utricularia bladder in section

The bifid glands are generally found immediately below and inside the trap entrance. Above: transverse sections of Utricularia bladders. Leftmost - when primed the bladders side-walls cave in (they are concave) as the fluid inside the bladder is at lower pressure than the water outside. Rightmost - an activated trap - water has rushed in and stretched the walls of the bladder which are now convex (curve outwards).

utricularia trap detail

Above: e: external glands, i: internal glands: b: bifid, q: quadrifid; s: stalk; T: trichomes; V: valve.

Below: structure of a quadrifid or bifid gland, shown in section. (Based on the model of Fineran, 1985).

The internal glands consist of an elevated basal cell, which is part of the epidermis (cell layer covering the wall of the bladder). Above this is a pedestal cell and above this are 2 (bifid gland) or 4 (quadrifid gland) capital or terminal cells. Black and dark shading indicate water-impervious regions of the cuticle and cell walls respectively. These ensure that water can only enter or leave the gland through the terminal cells (which lack any substantial cuticle) and also block the apoplastic pathway between the basal and pedestal cells.

Mechanism

These glands pump out water from the inside of the trap when it is being primed, lowering the pressure in the trap and creating the suction when the trap springs open. This process is rapid and so probably occurs through the apoplast (cell walls) as shown by the blue arrow. The movement of this water is actively driven. Pumps in the cell-surface membrane of the pedestal cell use cellular energy to pump water from the apoplast across the membrane into the cytoplasm of the pedestal cell. The pedestal cell is a type of transfer cell and has tubular wall-ingrowths which increases the surface area of its cell-surface membrane to accommodate more pumps. These pumps are membrane-spanning proteins that pump chloride ions. (Chloride ions are negatively charged and their charges attract and drag positively charged sodium ions with them). This increases the salt concentration inside the cytoplasm of the pedestal cell, lowering its water potential. Water then follows by osmosis (drawn out by the salt) as it moves from higher to lower water potential. This water is replaced by water from the apoplast and a stream of water movement through the apoplast is set-up. Water then moves from the pedestal cell into the basal cell and surrounding tissues through the symplast (cell cytoplasms) moving from cell to cell through pores called plasmodesmata.

The terminal cells possibly also secretes the enzymes to digest the prey when the trap is activated. The products of digestion are absorbed by the terminal cells and are thought to follow the symplast route (movement in the cytoplasm) moving from cell to cell through the plasmodesmata.

Once the trap has done its job, it is reset and can be used repeatedly. The water that is pumped out every time the trap is reset has to be removed from the bladder and this is the function of the external glands.

External gland/trichome

The external glands/trichomes consist of two cells - the pedestal cell and the terminal cell. Their function is to excrete excess water taken up from inside the trap when it is reset. Water absorbed by the external glands enters the tissues of the bladder wall and is transported from cell to cell through the plasmodesmata until it reaches the external glands where it enters the pedestal cell through plasmodesmata. The pedestal cell has tubular wall ingrowths (though not as extensive as in the internal glands). These ingrowths increase the surface area of the cell-surface membrane follows them passively by osmosis. The cuticle of the terminal cell is highly porous (especially when stretched by water entering the terminal cell) and the water passes out through these pores.

The  relative lengths of and angles between the arms of the quadrifid glands are important taxonomic characters in identifying species of
Utricularia.


Venus' Flytrap (Dionaea muscipula)

Similar to the bladderwort, the flytrap is a spring trap. Dionaea produces modified leaves as traps, with each trap comprising a pair of valves. Each valve is fringed with hairs called cilia and has three trigger hairs, arranged in a triangle, on its inside surface. Each plant produces 5-7 traps which are each 3-7 cm long.

Venus' Flytrap

Alluring glands inside the trap secrete a sugary mucilage to attract insects. When an insect touches one of the trigger hairs electrical signals are generated. Initially these are local signals which decay rapidly over time and space. A single brush does not activate the trap, otherwise many a false alarm such as a falling raindrop or debris could falsely trigger the trap, but if any of the three hairs is touched again, then it generates a second electrical signal which is added to the first. In this way, multiple touches that are close enough together in time will reach a threshold and the trap will be activated. This process involves the transport of electrical signals throughout the plant neuroid/nervous system (see the sensitive plant).

When cells in the trap receive the signal, especially the hinge and central areas of the valves where most movement occurs, then motor cells reversibly close the trap. The mechanism of closure is very different to the mechanism that moves leaves to track the Sun - the pulvinus, or hinge joint at the base of a leaf stalk, has flexor and extensor cells that swell and shrink reversibly as they take-up or lose water. In contrast, the movements that cause trap closure in the flytrap are irreversible growth changes - flexor cells grow permanently larger. This growth appears to be triggered by the active pumping of hydrogen ions out of the flexor cells and into the cell walls, lowering the pH of the apoplast (cell wall compartment). This acidification of the cell walls activates wall-loosening enzymes, called
expansins, which break cross-links between the cellulose microfibrils (microscopic fibres of cellulose) that make up the cell wall, allowing them to slide past one-another and allowing the cell walls to stretch. As the cell walls looses, so the cells take in water and swell irreversibly. Movement of water from the inner half to the outer half of the hinge provides the necessary water for the flexor cells in the outer part of the hinge. This is a rapid example of the normal cellular growth process. Pumping of the hydrogen ions is an energetic process and the cells use about 30% of their ATP (energy store) in this process.

Trap closure occurs in several stages:

1. Slow initial phase lasting 0.3s during which the trap only closes by 20%. During this phase, rapid growth of cells is building up elastic energy which is stored inside the trap.
2. Rapid closure phase lasting 0.1s, during which time the trap closes by 60%. This is the phase that traps the
unfortunate insect before it has time to escape.
3. Second slow phase, during which the trap closes by the remaining 20% and lasting 0.3s.

Following closure is the appression phase, lasting about 30 minutes and during which the trap presses around its prey and smothers it in secreted mucilage. Following this is the sealing phase, during which the edges seal tightly to prevent leakage of contents. This phase lasts for about one hour and is followed by enzyme secretion. Note that the trigger hairs are hinged and so fold back out of the way during trap closure.

The hairlike cilia seal the edge of the trap after closure, but allow small prey to escape, since it is a waste of time the plant digesting such small prey. If small prey escapes through the cilia, then the trap will soon open again (within 2 days). Larger insects, however, cannot escape and the edges of the trap then more slowly form a tighter seal. Secreted mucilage also entraps the insect and when the seal is formed, digestive enzymes are secreted and the prey liquefied and absorbed. Digestion takes 4-14 days and the digestion glands give the inside of the trap its red colour (due to anthocyanin pigment), which probably helps to attract insects. Extensor cells then grow larger and reopen the trap, resetting it. Despite the fact that the growth changes are  irreversible, successive growth cycles are possible and the trap can be sprung and reset several times in its lifetime.

Venus Flytrap

Charles Darwin in his 'Insectivorous plants' (1875, (17)) carried out thorough observations and experiments on feeding behaviour in Dionaea muscipula. Some of his key observations are useful to plant growers and botanists alike and are listed below:

Venus Flytrap

Above: note the triangle of sensory bristles on each valve of the trap. Usually there are 3 bristles per valve (but sometimes 2 or 4). The redness inside the trap is due to the production of anthocyanin pigment. Anthocyanin synthesis is generally considered a stress response to high light intensity in flowering plants. This does not harm the plant, however. This individual plant was originally very green when bought and still very healthy, but the inside of the traps flushed red on exposure to direct sunlight. If a flytrap is too shaded then its leaves will etiolate and develop thin elongated leaves (21).

Some varieties naturally produce more anthocyanins than others, even in lower light intensities. In those forms that do produce the anthocyanin in response to light, plants illuminated enough to produce red traps are reported to grow better than those under-illuminated plants that stay green. Some varieties however remain yellow-green, being unable to synthesize the shielding anthocyanins, and when these cross with the red-trap varieties they produce individuals of various redness, including varieties like this one in which the center of the trap lobes flush red but never become deeply red.

Venus Flytrap flower

The flower of Venus's Flytrap. Flytraps flower in April, shortly after leaving winter dormancy, but may continue flowering into July (21). The main inflorescence stalk or scape (peduncle arising from a subterranean stem) may reach 40 cm in length and terminates in a series of bracts, each bract subtending a pedicel bearing a single flower. Each inflorescence may consist of up to 40 flowers which open one at a time at intervals of 24 to 48 hours. The flowers are protandrous: the male organs ripen first with pollen being shed shortly after the flower opens. The stigma then becomes receptive 24 to 48 hours later. This reduces the likelihood of self-pollination. The flower then withers as the next flower opens.

Venus Flytrap flower

Each of the five petals is up to 15 mm long and there are 5 green sepals. There are up to 15 stamens and a style terminating in a stigma fringed with hairs. The superior ovary is green and encloses a single chamber. The fruit is a capsule containing up to 40 shiny black seeds. The capsule lid shrivels away, sometimes explosively, to expose the seeds for dispersal. (21)

It is often said that flowering may weaken a Dionaea plant, especially in cultivation. I don't think there is much well documented evidence for this, mostly anecdotes and this seems to be a controversial topic. Plants often alternate their allocation of resources between flowering and vegetative growth and so it is reasonable to expect a reduction in leaf growth while flowering occurs: plants allocate their resources intelligently. Plants may also abort some of the flower buds if resources are limiting. This particular individual has so far flowered the past two springs after leaving winter dormancy. I think carefully controlled cultivation experiments are needed to test the effects of flowering on healthy plants.


Genlisea - mouths, stomachs and all!

Genlisea rhizophyll

Above: a rhizophyll of Genlisea, showing the vesicle (stomach) and the spiral arms with their slit-like openings. The main mouth is in the junction of the two arms and hidden from view.

These small and remarkable rosetted herbs of South America and Africa have spatulate (spatula-shaped: having a narrowed base and expanded tip) leaves lying on or close to the soil surface (epiterrestrial). Genlisea has no roots, but remarkably has modified root-like subterranean leaves, lacking in chlorophyll, called rhizophylls, which form subterranean traps for tiny soil organisms. The rhizophylls are tubular: the original adaxial (upper) surface of the leaf forms the inside of the tube as the ancestral leaf has been rolled into a tube joined along the suture. Each rhizophyll is constructed as follows: a short basal trap stalk (footstalk) widens into a hollow vesicle ('stomach') forming the digestion chamber. This is followed by a narrow tubular neck which widens and forks into two branches (trap arms) which are helically spiralled. The whole is roughly Y-shaped. In the region between the bases of the two branches is the main mouth, though additional smaller slit-like mouths are arranged along the suture of the twisted trap-arms.

Bristles line the inside of the trap neck and arms and these are angled to point towards the stomach and presumably serve to prevent prey from escaping. Multicellular glandular hairs line the vesicle and secrete the digestive fluids. Prey includes mites, nematodes, protozoa, small algae, which are ingested along with soil particles and other debris. The presence of soil particles and debris suggests that the traps are not passive and simply do not wait for organisms to enter. Indeed, it has been shown experimentally that water-currents enter the traps, which presumably drag other materials and organisms with them. The suction mechanism is not understood, but may be linked to transpiration (detached rhizophylls lose their suction power - they need to be attached to the whole plant in order to work).

Some species have an additional elaboration: they exhibit trap dimorphism. They have larger and deeper traps and smaller, more horizontal and shallower traps. These different trap types are presumably specialized to catch different types of organism.

Semi-Carnivorous Plants

Many plants are not classed as fully-carnivorous but still 'eat' insects and make use of the nitrogen as a nitrogen supplement. One such example of a semi-carnivorous plant is the teasel. To be considered a fully carnivorous plant, three criteria must be satisfied (21):

  1. The plant must attract animal prey.
  2. Trap animal prey.
  3. Derive benefit from the prey.

The teasel certainly has specialized leaves that trap animal prey and has been shown to derive considerable nutritive benefit. However, it is not clear whether the traps lure prey. Many plants are able to derive nutriment from trapped animals.


Sarracenia Pitcher Plants (Trumpet pitchers)

Parrot Pitcher Plant

Above and below: Sarracenia psittacina, the Parrot Pitcherplant (Chelsea Physic Garden). The pitchers of this plant resemble those of Sarracenia minor except they are prostrate, forming a prostrate rosette whereas those of S. minor are erect. The hoods of S. psittacina also tend to form beak-like protuberances and look more parrot-like.

The traps of S. minor have hoods that hang down over the openings and translucent windows at the back. insects attracted to the nectar secreted by the trap rim and the edge of the wing (ala) which leads up to the rim, are attracted to the light passing through the windows so when they attempt to leave and fly away into the sky they aim for the windows instead and hit the back of the hood and slide down into the trap, which gets narrower towards the bottom, making it increasingly difficult for insects to reverse. Eventually they become trapped in the digestive fluid at the base of the pitcher.

Sarracenia psittacina has small openings and windows and backward-facing hairs make it hard for an insect that has entered the trap to escape. The prostrate traps are frequently submerged by water and catch arthropods and tadpoles and works more like a 'lobster pot' trap whereas the upright pitchers of Sarracenia minor operate as pitfall traps. The Parrot Pitcherplant sometimes also produces semi-erect pitchers with reduced wings (alae).

Parrot Pitcher Plant

There are 8 to 11 recognised species of Sarracenia and these plants span the subtropical, temperate and Arctic zones of North America. Most occur in the USA, but the Northern Pitcherplant, Sarracenia purpurea, extends into Canada. Sarracenia minor is much more southern and grows in bogs in openings in pine forests in North Carolina, South Carolina, Georgia and Florida (regions that are chiefly subtropical). (External link: Botanical Society of America).A giant form of S. minor, the Okefenokee Giant variety, var. okefenokeensis also occurs. Sarracenia psittacina occurs from Georgia to Louisiana again in subtropical pine forests. Similarly, a gigantic form of S. psittacina, var. okefenokeensis has pitchers reaching over 30 cm in length.

The plant below was purchased from Chelsea Physic Garden, London (without reading the labels carefully). It is clearly a hybrid and after careful consideration I have decided that is most probably Sarracenia x Swaniana. Sarracenias hybridise easily and hybrids may consist of 2 or more parent species in differing proportions. The hybrid Swaniana is a cross between Sarracenia purpurea and S. minor (Sarracenia purpurea x S. minor).

Sarracenia X Swaniana Pitcher Plant

Note the drops of nectar on the edge of the ala and the rim of the trap and the tracts of downward-pointing hairs on the inside of the hood. The shape of the hood and the tracts of hairs are very suggestive of Sarracenia purpurea as is the wavy ala. Note also the translucent windows, suggestive of S. minor. (The transparent trays were eventually replaced with opaque ones since they encouraged the growth of cyanobacteria and it was not sure what effect nitrogen fixation and secretion of other materials by these organisms into the water would have on the pitcherplant, although there are lower numbers of cyanobacteria in the surface soil of course).

Sarracenia X Swaniana Pitcher Plant

The pitchers slowly reddened at their top ends whilst sitting on the windowsill, beginning with the veins. This red coloration is due to the pigment anthocyanin. The amount of anthocyanin produced depends on the species and variety and also on exposure to light and perhaps heat. The anthocyanins are produced when the plant is stressed by excessive radiation and help to protect the chlorophyll against bleaching. This does not necessarily mean that a red plant is unhealthy since plants that often grow in the open must shield themselves from UV light and in some plants, those anticipating very bright sunlit conditions, anthocyanin synthesis is often constitutively switched on so these plants always tend to be reddish. By the same token, reddness is not necessarily an indication that the plant is getting 'enough' light since this plant was mostly green when purchased but evidently very healthy.

This plant is growing near to open windows in the summer and so has caught some of its own food, though this has been supplemented by rehydrated mealworm pieces. the large tray is constantly filled with deionised water to about an inch in depth. This generates a moist enough atmosphere to prevent the tops of the pitchers from drying out (a problem that occurred early on and was quickly remedied). Not all deionised waters appear to be suitable, however, and when a good supply of water is found I think it's advisable to stick with it. Although this water has neutral pH, it has been claimed that the plants may prefer slightly acidic water as would be found in their natural bog/fen/seepage marsh habitats. this particular hybrid appears to grow well indoors and once the conditions are right then it will grow vigorously, but if anything is not quite right then the pitchers and roots will quickly begin to die back. Sarracenias do have particular requirements but grow vigorously once conditions are suitable.

Sarracenia X Swaniana Pitcher Plant

Note that the developing leaves are mostly blade (ala) with no traps to begin with no trap, then the trap cavity develops and eventually the trap opens and the hood expands. The smaller traps are not developing traps but mature small traps and are a deliberate part of the plant's growth strategy. However, larger traps generally catch more prey, though studies in Sarracenia pitchers have shown that in nature as many as 50% of pitchers may fail to catch anything. The newer pitchers began to produce copious nectar, which may reflect the ample light for photosynthesis or the age of the traps or perhaps the plant was not catching enough insects. The large frilly openings in the larger pitchers is a character inherited from S. purpurea. Some of the smaller traps appear more covered as are the traps in S. minor.

Sarracenia purpurea forms large sac-like traps with large opening that are not covered. These traps will catch falling debris in addition to insects and other small invertebrates and micro-organisms. There is some controversy over whether or not this species produces digestive enzymes or relies on digestion carried out by the microbial and invertebrate communities that accumulate in the open traps and simply absorb what is produced by them. However, research has clearly shown that although individual traps may persist for two years in this species, they catch most of their prey within the first 50 days after opening. Within the first two weeks liquid within the traps contained digestive enzymes (proteases, nucleases and phosphatases) even though the microbial count was very low. This suggests that traps of Sarracenia purpurea do indeed synthesise some of their own enzymes, especially early on. Although this enzyme secretion switched off later on, the necessary genes would switch back on if arthropods (or raw nutrients) were introduced into the traps. However, older traps do indeed have communities of microbes and support the development of the larvae of specialised species of insect, such as the Mosquito Wyeomyia smithii and the moth Endothenia daekeana. They also house the copepod Paracyclops canadensis (which is rare elsewhere). These organisms grow and develop inside the liquid of the pitchers without being digested, suggesting that older traps either eventually lose their ability to produce digestive enzymes or that the inhabitants send some signal to switch them off or that the enzymes are fended off by living organisms, e.g. by lipid or mucus secretion. Parts of plants that trap water that may house unique ecosystems are called phytotelmata (sing. phytotelma).

Sarracenia X Swaniana Pitcher Plant

The leaf form on individual Sarracenia plants is subject to environmental regulation. Some of the leaves produced are phyllodia in which the keel (wing or ala) becomes the prominent feature of the leaf with the trap itself reduced or absent. Note that 'phyllodia' should not be confused with 'phyllode' which is a flattened leaf-like petiole or leaf-stalk found in some plants though sometimes the phyllodia of Sarracenia is also termed a phyllode, it most probably incorporates much of the leaf blade into its structure (only developmental studies can really answer this type of question). The plural of phyllodia is phyllodia or phyllodiae; the singular is either phyllodium or phyllodia: usage varies. There is a continuous spectrum from leaves which are predominantly traps to leaves which are predominantly phyllodiae. Experiments in Sarracenia purpurea have shown (12) that supplying the plants with more nitrogen increases the proportion of leaves which are phyllodiae. This makes sense: the leaves are important organs of photosynthesis and producing traps is expensive so when nitrogen is plentiful leaves develop into phyllodiae.It should be noted that the juvenile leaves of Sarracenia begin as phyllodiae with the traps expanding and opening as the leaf matures, which suggests that when nitrogen is plentiful some of the genes for leaf maturation are switched off, leaving the leaves to continue growth with juvenile characters. Of course the leaf evolved before the pitcher and the developing leaf repeats this trend, to some extent, as it grows and so begins much as a normal leaf with the trap developing and opening later.

Sarracenias are perennials, overwintering largely as an underground rhizome, though the leaves in Sarracenia purpurea are evergreen and last for over a year. In most species the pitchers die back over winter. The amount of short days and cold nights that a pitcherplant needs to induce its overwintering dormant state obviously depends on the species and its natural latitude. The Swaniana hybrid here has one parent from the subtropics and one from the arctic, so it will be interesting to see how it manages. Sarracenia purpurea and S. flava have been planted out in several wild locations in Britain and are apparently doing well.

Sarracenia X Swaniana Pitcher Plant

The flower of Sarracenia hangs upside-down on a long pedicel. There are three bracts, five sepals and five petals. the style expands out into an umbrella which traps pollen that falls from the numerous stamens. Insects entering the flower, perhaps seeking refuge, may pick up some of this pollen. The umbrella is pentamerous (with five-fold radial symmetry) and bears stomata and is photosynthetic and bears five radial veins. Near the tip of each vein is a small projection inside the umbrella which is the stigmatic surface. The petals are arranged such that insects can only enter the flower by passing past a stigma and they may deposit any pollen they are carrying here. The five veins contain pollen-tube conducting tissue as well as vascular tissue. the conducting tissue forms a hollow tube for the pollen tube to grow through, guiding it to the ovules inside the ovary, a distance of about 4 cm. On this long journey the umbrella must nourish the growing pollen tube, which is presumably assisted by the fact it carries out its own photosynthesis (14).

Sarracenia flower diagram

Sarracenia flower diagram

Sarracenia flower diagram

In S. minor and S. oreophila, flowering occurs at the same time as trap production, but generally in Sarracenia, flowering in spring occurs for several weeks before trap production to prevent pollinators from becoming prey! (20)


Phyllodia and Pseudo-phyllodia

Below: phyllodia (sometimes phyllodiae is used for the plural and phyllodium for the singular) produced by the Sarracenia purpurea X S. minor hybrid discussed above. These leaves have proportionately better developed blades and smaller (generally nonfunctional) trap structures. Sarracenia produces phyllodia under two conditions: 1) low light levels (these two were the first two leaves to emerge after winter dormancy on this specimen but similarly they may also appear in autumn) and 2) when nitrogen levels are sufficient (15, 16). In other words they help the plant increase photosynthesis and the plant will not waste resources on producing traps until traps are needed. These phyllodia grew vertically initially, but since the trap structure does not develop and thicken they curved over to become prostrate, which is normal for large phyllodia. Traps normally pass through a small phyllodium stage as they develop, but in the case of phyllodia the trap development is inhibited and the mature leaf retain juvenile features.

Sarracenia flower diagram

Another factor affecting leaf morphology in Sarracenia is light. McPherson and Schnell (2011, Sarracenia of North America, Redfern Natural History Productions) make a distinction between true phyllodia and pseudophyllodia and suggest there is much confusion between the two. In this scheme, true phyllodia, e.g. of Sarracenia flava, completely lack a pitcher cavity and are sword or sickle-shaped and usually upright. In contrast pseudophyllodia are pitchers formed under stress conditions, such as insufficient illumination. These 'etiolated pitchers' have reduced pitcher tubes and domes and expanded wings. The phyllodia shown here look more like pseudo-phyllodia: note the small partially formed opening at the tip.

This plant was recently moved to a new location and was indeed showing signs of stress and has since been moved to a third location with a bit more light to see if this helps. I have no ideal place for it (outdoors or a window with more hours of sunlight would be better) and so I am doubtful this plant will thrive. It has at least gone through two winter dormancies and is still alive, however, so we shall see. When placed in brighter conditions this plant produced a number of true upright phyllodia and a normal trap.

The production of true phyllodia at certain times of the year, in addition to traps of different sizes, is part of the normal annual growth cycle for some species. For example, S. minor may sporadically produce a small number of phyllodia, whereas S. psittacina and S. purpurea normally produce no phyllodia at all. In contrast, S. leucophylla and S. rubra first produce small spindly traps in spring, sword-shaped phyllodia over the summer and larger and sturdier traps in late summer and autumn. (20)

There is perhaps a spectrum of leaf morphologies from phyllodium to pitcher, with true phyllodia being the most leaf-like and pseudo-phyllodia intermediate. There is also a possible difference in function, however. McPherson and Schnell describe true phyllodia as being normally produced by some species in addition to normal pitchers with the phyllodia persisting overwinter. A group of upright leaves which could be phyllodia or pseudo-phyllodia (hard to tell) persisted on the plant shown here over winter, whilst the prostrate leaf shown died away (its prostrate habit does suggest etiolation - the base become too thin to support the leaf).

Sarracenia flower diagram

It should be noted that it has been shown that the roots of Sarracenia purpurea can absorb amino acids directly (18). The presence of inorganic nitrogen increases the proportion of leaves that develop as phyllodes, so the roots still play an important part in N acquisition. under some conditions, carnivory may supply as little as 10% of the plant's nitrogen (15). The relative importance of the roots versus the traps in the mineral nutrition of the carnivorous plant varies considerable with species.

Roots

In many carnivorous plants the roots are either poorly developed (accounting for an unusally small proportion of the plant's biomass) or absent in aquatic forms (19). These roots are usually short, weakly branched or unbranched but regenrate easily. Despite being in waterlogged bog-soil these roots generally have a low air space volume without extensive development of aerenchyma seen in the roots of many aquatic plants or plants of waterlogged soils (such as in Salix (willow), Phragmites (reeds) and Carex (sedges)). Some, such as Darlingtonia, do have an extensive system of small air-spaces in the root cortex, however, and the rates of respiration in the roots of carnivorous plants is generally high regardless. Subterranean parts are important in surviving fires (that may clear away competitors) in Darlingtonia californica, some Drosera and Dionaea muscipula. Utricularia lacks roots but has specialised shoots (colourless shoots and rhizoids) that take over the functions of roots.


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Article last updated:

17 March 2015
17 Dec 2015
22 Sep 2019
17 Apr 2020
04 Apr 2021

23 Apr 2021